Relatively low-power (e.g., 10 dB above noise floor) auxiliary signals encoding digital information can be admixed together with composite video signals without being objectionably evident in television pictures generated from those composite video signals, if suitable restrictions on the digital signal format are observed. It is advantageous to use a suppressed, vestigial-sideband, amplitude-modulated (VSB AM) carrier of the same frequency as the VSB AM picture carrier, but in quadrature phasing therewith, for transmitting the digital data. This procedure permits the synchronous detection of the modulation of that quadrature carrier to recover the digital data. If the bandwidth of the receiver is sufficient to include the entire vestigial sideband, remnant composite video signal accompanying the digital data as an interfering signal will not have substantial energy in the baseband extending up to 0.75 MHz in frequency. It is around 0.75 MHz that the VSB AM video carrier begins the transition from being a double-sideband amplitude-modulated (DSB AM) carrier to being a single-sideband amplitude-modulated (SSB AM) carrier, and at lessened energy up to the 1.25 MHz frequency at which roll-off of the vestigial sideband is complete.
A. L. R. Limberg, C. B. Patel and T. Liu in their U.S. patent application Ser. No. 08/108,311 filed 20 Aug. 1993, entitled APPARATUS FOR PROCESSING MODIFIED NTSC TELEVISION SIGNALS, WITH DIGITAL SIGNALS BURIED THEREWITHIN, and incorporated by reference herein describe phase-shift-keying (PSK) modulation of a subcarrier of the VSB AM carrier that is in quadrature phasing with the VSB AM video carrier of the same frequency. The frequency of their subcarrier is an odd multiple of one-half scan line frequency, and it is phase-shift-keyed in accordance with serial-bit digital data supplied at a symbol rate that is a multiple of scan line frequency. Limberg et alii prefer transmitting frames of the modulated subcarrier twice, but in opposite phasing in each successive pair of consecutive frames of the NTSC television signal. Because of frame-averaging effects resulting from the limitations on the speeds of the response of the human visual system and the decay of electroluminescence of kinescope phosphors, such repetition of data in pairs of frames makes PSK subcarrier accompanying the composite video signal detected from the NTSC television signal less visible in images that are generated from the composite video signal for viewing on a screen. Such repetition of data in pairs of frames also provides a basis for using frame-comb filtering in a digital signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal that describes static portions of successive television images. Limberg et alii prefer also repeating the modulation of the digital data in antiphase in contiguous pairs of adjoining scan lines of the NTSC television signal, providing a basis for using line-comb filtering in the digital signal receiver to separate PSK subcarrier from the chrominance portion of the composite video signal.
Limberg et alii describe a digital signal receiver in which the synchronous video detector for quadrature-phase VSB AM video carrier is followed by a cascade connection of a lowpass line-comb filter and a highpass frame-comb filter. The lowpass line-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from chrominance signal portions of the frequency spectrum of an NTSC signal, particularly of an NTSC signal that has been appropriately pre-filtered. The highpass frame-comb filter is for separating the frequency spectrum of a PSK subcarrier having a frequency that is an odd multiple of half-scan-line frequency from motion-free luminance signal portions of the frequency spectrum of an NTSC signal. Limberg et alii teach that the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be viewed as the frequency spectrum of a jamming signal accompanying the PSK signal. Accordingly, the remnant spectrum of the NTSC signal in the response of the cascaded highpass comb filters can be discriminated against by synchronous symbol detection.
U.S. patent application Ser. No. 08/141,070, filed 26 Oct. 1993 by J. Yang, entitled APPARATUS FOR PROCESSING NTSC TV SIGNALS HAVING DIGITAL SIGNALS ON QUADRATURE-PHASE VIDEO CARRIER and incorporated herein by reference, describes binary phase-shift-keyed (BPSK) modulation of a suppressed carrier that is the same frequency as a video carrier and is in quadrature phasing therewith. The suppressed carrier is phase-shift-keyed directly, without any subcarrier being used. Yang also advocates transmitting frames of the modulated subcarrier twice, but in opposite phasing in each successive pair of consecutive frames of the NTSC television signal, just as Limberg et alii do. Yang advocates the BPSK signals being constrained to about 2 MHz bandwidth, so as to avoid crosstalk into chroma in TV receivers that separate chroma from luma without recourse to comb filtering. Yang indicates a preference for passing the data to be transmitted through a pre-line-comb partial-response filter prior to its digital-to-analog conversion to an analog modulating signal for a balanced amplitude modulator. This is done to preserve the information contained therein when line-comb filtering is done in the digital signal receiver to separate PSK subcarrier from the luminance portion of the composite video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to ternary digital signal, if the line-comb filtering is of the two-tap type, linearly combining signals differentially delayed by only the duration of one horizontal scan line of video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to five-level digital signal, if the line-comb filtering is of the three-tap type, linearly combining signals differentially delayed by the duration of one horizontal scan line of video signal and by the duration of two horizontal scan lines of video signal. Therefore, multi-level symbol decision circuitry is required to recover bit-serial digital data transmitted by the BPSK from the comb filtering response.
U.S. patent application Ser. No. 08/179,616 filed 5 Jan. 1994 by J. Yang and A. L. R. Limberg, entitled "PRE-FRAME-COMB" AS WELL AS "PRE-LINE-COMB" PARTIAL-RESPONSE FILTERING OF BPSK BURIED IN A TV SIGNAL and incorporated herein by reference, describes the digital signal transmitter using a, pre-frame-comb partial-response filter as well as pre-line-comb partial-response filtering for processing bit-serial data from which BPSK modulating signal is generated for the carrier in quadrature phasing with the video carrier. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to five-level digital signal, if the line-comb filtering is of the two-tap type, linearly combining signals differentially delayed by only the duration of one horizontal scan line of video signal. Line-comb filtering in the digital signal receiver converts the partial-response filtered binary digital signal to nine-level digital signal, if the line-comb filtering is of the three-tap type, linearly combining signals differentially delayed by the duration of one horizontal scan line of video signal and by the duration of two horizontal scan lines of video signal.
U.S. patent application Ser. No. 08/179,588 filed 5 Jan. 1994 by J. Yang and A. L. R. Limberg, entitled APPARATUS FOR PROCESSING BPSK SIGNALS TRANSMITTED WITH NTSC TV ON QUADRATURE-PHASE VIDEO CARRIER, and incorporated herein by reference, describes BPSK modulating signal for the carrier in quadrature phasing with the video carrier being generated directly from bit-serial data without any pre-comb-filter partial-response filtering. The same patent application describes digital signal receivers, which use a cascade connection of a highpass frame-comb filter and a highpass line-comb filter after the quadrature video detector to suppress interfering remnant luminance signal, which use plural-level symbol decision circuitry for the comb filter response, and which use post-comb-filter partial-response filtering after the symbol decision circuitry for undoing the data alteration caused by the comb filtering.
Receivers for the Yang system are also described by T. V. Bolger in his U.S. patent application Ser. No. 08/141,071, filed 26 Oct. 1993, entitled RECEIVER WITH OVERSAMPLING ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS WITHIN TV SIGNALS, and incorporated herein by reference. These receivers digitize the response of a quadrature-phase video detector using an oversampling analog-to-digital converter. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level symbol decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein.
Receivers for the Yang system are also described by J. Yang, T. V. Bolger and A. L. R. Limberg in their U.S. patent application Ser. No. 08/179,586 filed 5 Jan. 1994, entitled RECEIVER WITH SIGMA-DELTA ANALOG-TO-DIGITAL CONVERSION FOR DIGITAL SIGNALS BURIED IN TV SIGNALS, and incorporated herein by reference. These receivers digitize the response of a quadrature-phase video detector using an oversampling analog-to-digital converter of sigma-delta type. Preferably, the bit resolution of a basic multiple-bit-resolution flash converter is improved by using a sigma-delta procedure in which only a single bit of the basic multiple-bit-resolution ADC output signal is converted back to analog signal for feedback purposes during each oversampling step, as described by T. C. Leslie and B. Singh in their paper "An Improved Sigma-Delta Modulator Architecture", 1990 IEEE SYMPOSIUM ON CIRCUITS & SYSTEMS, 90 CH 2868-8900000-0372, pp. 372-375, incorporated herein by reference. The digitized quadrature-phase video detector response is subjected to digital frame-comb and line-comb filtering to suppress remnant composite video signals; the comb filtering response is supplied to multi-level symbol decision circuitry to recover bit-serial digital data transmitted by the BPSK; and the bit-serial digital data is supplied to a decoder that corrects the digital information in the data using forward-error-correcting codes contained therein.
U.S. patent application Ser. No. 08/179,618 filed 5 Jan. 1994 by C. B. Patel and J. Yang, and entitled APPARATUS FOR SUPPRESSING 30 GHOSTS IN SIGNALS MODULATING A CARRIER IN QUADRATURE PHASING WITH A VIDEO CARRIER, and incorporated herein by reference, describes the use of similar ghost-suppression filters for signals from in-phase and quadrature-phase video detectors in a digital signal receiver. The filtering coefficients of the two ghost-suppression filters are adjusted in parallel, responsive to calculations made from the in-phase video detector response to ghost-cancellation reference (GCR) signals broadcast during selected horizontal scan lines in the vertical blanking interval of the NTSC composite video signal.
The inventions described in the patent applications referred to above, like the inventions described herein, are assigned to Samsung Electronics Co., Ltd., pursuant to pre-existing employee agreement so to assign inventions made within the scope of employment. Certain modifications to the systems described in these applications have been subsequently considered. As originally described these systems include data in all horizontal scan lines, including all horizontal scan lines in the vertical blanking interval, and data frames are started after the vertical sync pulse interval. Alternatively, data frames can begin with the 22.sup.nd horizontal scan line of each odd field of composite video signal with data not being transmitted during the 18.sup.th through 21.sup.st lines of each field of composite video signal. This practice avoids any changes with regard to the 19.sup.th lines being used for ghost cancellation reference (GCR) signals, the 20.sup.th lines being used for video facsimile transmissions, and the 21.sup.st lines being used for closed caption information.
The bandwidths available from the systems described in the patent applications referred to above accommodate the transmission of 5.1-channel Dolby AC-3 audio or MPEG audio.
A previous preference was to use the -20-1RE-level midpoints in edges (usually the leading ones) of the horizontal sync pulses as the leftmost edge of the first pixel in each horizontal scan line and to reckon all pixel clocking from those points. A problem that arises when considering reference points for data symbol clocking is that the NTSC television standards do not specify close tolerances on the time relationships between horizontal sync pulse edges, color burst and GCR signals. Since in the digital-signal receiver the GCR signal is stored on a point-by-point basis in read-only memory (ROM), for use by the processor used for calculating adjustments of the weighting coefficients of ghost-suppression filtering, preferable transmitter practice is to refer the transmitted GCR signal and the transmitted symbol clock to each other. Then, the ghost-suppression filtering in the digital-signal receiver can automatically provide for the adjustment of symbol clocking phase, as the processor seeks to cross-correlate the GCR signal received from the transmitter with the GCR signal stored in its ROM. This practice is also preferable from the standpoint that the transmitted GCR signal is usually locally generated at the transmitter and so does not depend on remotely originated composite video, which may have the timing relationships between horizontal sync and color subcarrier altered by differential phase distortion.
In a digital signal receiver for receiving digital signals buried in conventional analog television signals, there are advantages to detecting the composite video signal modulating the amplitude of the VSB video carrier using an in-phase video detector, in addition to a quadrature-phase video detector for recovering digital information. The synchronizing pulses for the composite video signal contain a large amount of useful timing information, which can be used to define data frames, data rows, and approximate PSK symbol positions. This timing information can also be used for controlling frame-comb and line-comb filtering of the signal detected by the quadrature-phase video detector, in order to suppress interfering remnants of the composite video signal. These remnants are above the 0.75 MHz frequency where the VSB AM video carrier begins the transition from being a double-sideband amplitude-modulated (DSB AM) carrier to being a single-sideband amplitude-modulated (SSB AM) carrier, exhibiting increased energy up to the 1.25 MHz frequency at which roll-off of the vestigial sideband is complete. The ghost cancellation reference (GCR) signals transmitted in the 19.sup.th scan line of each field provide information concerning a modulo-8 field (or half-frame) count that is useful in relating frames of data to each other. Since an in-phase video detector is advantageously included in a digital signal receiver, anyway, the GCR signals it detects from the 19.sup.th scan line of each field are available as a basis for calculating multipaths in the transmission channel.
The bit rate for the BPSK has to be a multiple of horizontal scanning frequency in order to implement partial-response filtering, done either at the transmitter or in the digital signal receiver, and to implement the separation of the BPSK from composite video signal by line- and frame-comb filtering in the digital signal receiver. The digital transmissions accompanying the television signals differ from ordinary digital transmissions, then, in that the accompanying television signals are relatively high energy signals sharing the same channel that tend to be jamming signals for the digital transmissions and so to interfere with recovery of the BPSK carrier. But at the same time, these strong interfering television signals contain timing information in their synchronizing pulses that is related to the frequency and phase of the BPSK carrier. Accordingly, it is advantageous to use the horizontal synchronizing pulses as input signal for the automatic frequency and phase control (AFPC) circuitry of a controlled oscillator used in the regeneration of clocking signals in the digital-signal receiver. Further, the GCR signals included in the 19.sup.th scan lines of the image fields of these strong interfering television signals provide the best reference signals for channel equalization purposes.